Point in Polygon

Description

pnt.in.poly works out if 2D points lie within the
boundaries of a defined polygon.

Note:
Points that lie on the boundaries of the polygon or
vertices are assumed to be within the polygon.

Usage

pnt.in.poly(pnts, poly.pnts)

Arguments

pnts

a 2-column matrix or dataframe defining
locations of the points of interest

poly.pnts

a 2-column matrix or dataframe defining
the locations of vertices of the polygon of interest

Details

The algorithm implements a sum of the angles made between
the test point and each pair of points making up the
polygon. The point is interior if the sum is 2pi,
otherwise, the point is exterior if the sum is 0. This
works for simple and complex polygons (with holes) given
that the hole is defined with a path made up of edges into
and out of the hole.

This sum of angles is not able
to consistently assign points that fall on vertices or on
the boundary of the polygon. The algorithm defined here
assumes that points falling on a boundary or polygon vertex
are part of the polygon.

Value

A 3-column dataframe where the first 2 columns are the
original locations of the points. The third column (names
pip) is a vector of binary values where 0 represents points
not with the polygon and 1 within the polygon.

Author(s)

Examples

#define the points and polygon
pnts = expand.grid(x=seq(1,6,0.1),y=seq(1,6,0.1))
polypnts = cbind(x=c(2,3,3.5,3.5,3,4,5,4,5,5,4,3,3,3,2,2,1,1,1,1,2),
y=c(1,2,2.5,2,2,1,2,3,4,5,4,5,4,3,3,4,5,4,3,2,2))
#plot the polygon and all points to be checked
plot(rbind(polypnts, pnts))
polygon(polypnts,col='#99999990')
#create check which points fall within the polygon
out = pnt.in.poly(pnts,polypnts)
head(out)
#identify points not in the polygon with an X
points(out[which(out$pip==0),1:2],pch='X')